Zymogen activation of neurotrypsin and neurotrypsin-dependent agrin cleavage on the cell surface are enhanced by glycosaminoglycans

The serine peptidase neurotrypsin is stored in presynaptic nerve endings and secreted in an inactive, zymogenic form by synaptic activity. After activation, which requires activity of postsynaptic NMDA receptors, neurotrypsin cleaves the heparan sulfate proteoglycan agrin at active synapses. The resulting C-terminal 22-kDa fragment of agrin induces dendritic filopodia, which are considered to be precursors of new synapses. Here, we investigated the role of glycosaminoglycans in the activation of neurotrypsin and neurotrypsin-dependent agrin cleavage. We found binding of neurotrypsin to the glycosaminoglycan side chains of agrin, which in turn enhanced the activation of neurotrypsin by proprotein convertases and resulted in enhanced agrin cleavage. A similar enhancement of neurotrypsin binding to agrin, neurotrypsin activation, and agrin cleavage was induced by the 4-amino-acid insert at the y splice site of agrin, which is crucial for the formation of a heparin-binding site. Non-agrin glycosaminoglycans also contributed to binding and activation of neurotrypsin and, thereby, to agrin cleavage albeit to a lesser extent. Binding of neurotrypsin to cell surface glycans locally restricts its conversion from zymogen to active peptidase. This provides the molecular foundation for the local action of neurotrypsin at or in the vicinity of its site of synaptic secretion. By its local action at synapses with correlated pre- and postsynaptic activity, the neurotrypsin-agrin system fulfi...

Source: BJ Cell - April 5, 2013 Category: Biochemistry Authors: C Gisler, D Lüscher, P Schätzle, S Dürr, A Baici, G Galliciotti, R Reif, M F. Bolliger, B Kunz, P Sonderegger Tags: BJ Cell Source Type: research